How to get the default value of any type

Question

In C# I can write something like this:

    class AnyThing
    {
        static public T Default = default(T);
    }

    static void Main ()
    {
              


        

Answer 1

ForEveR's answer will not work if T doesn't have a copy constructor. In C++03, there is no way to zero-initialize a variable that is both generic and elegant. All that's left is the following trick.

T temp[1] = {};
T & obj = temp[0];

Here, temp[0] is zero-initialized and then bound to obj. No copy constructors are needed.

Answer 2

Taken literaly from "The C++ Programming Language, Third Edition by Bjarne Stroustrup":

BEGIN QUOTE

4.9.5 Initialization [dcl.init]

If an initializer is specified for an object, that initializer determines the initial value of an object. If no initializer is specified, a global (§4.9.4), namespace (§8.2), or local static object (§7.1.2, §10.2.4) (collectively called static objects) is initialized to 0 of the appropriate type. For example:

int a;  // means int a=0;
double d; // meands d=0;

Local variables (sometimes called automatic objects) and objects created on the free store (sometimes called dynamic objects or heap objects) are not initialized by default. For example:

void f()
{
   int x;   // x does not have a well-defined value
   // . . . 
}

Members of arrays and structures are default initialized or not depending on whether the array or structure is static. User-defined types may have default initialization defined (§10.4.2).

More complicated objects require more than one value as an initializer. This is handled by initializer lists delimited by { and } for C-style initialization of arrays (§5.2.1) and structures (§5.7).

For user-defined types with constructors, function-style argument lists are used (§2.5.2, §10.2.3). Note that an empty pair of parentheses () in a declaration always means ‘‘function’’ (§7.1). For example:

int a[] = {1,2};    // array initializer
Point z(1,2);       // function-style initializer (initialization by constructor)
int f();            // function declaration

END QUOTE

So, you can get the default value of any type form a static object of that type:

static T defaultT; // `defaultT' has de default value of type T

Answer 3

In C++ there is no something like default keyword in C#. Since initialization by default constructor of value of class-type will be failed, if default constructor is private. In C#, if default constructor is private, value of class-type will be initialized to null, since class-type is reference-type.

Initialition by {} is defined by language specification. It's C++11. In C++03 you should use

T obj = T();

As pointed by bames53 in comment, when you want to initialize T* you should use

before C++11.

T* obj = 0;

or

T* obj = NULL;

in C++11.

T* obj = {};

or

T* obj = nullptr;

Answer 4

Create your own default keyword:

class default_t
{
public:
  template<typename T>
  operator T() const { return T(); }
};

default_t const default = default_t();

Use it like:

int myInt = default;
vector<string> myVector = default;
shared_ptr<string> myPtr = default;

Or with a slight semantic variation:

default_t const empty = default_t();

vector<Persons> fetchPersons()
{
  if (Database::isConnected())
  {
    return Database::fetchPersons();
  }

  return empty;
}